fix bigint twos complement implementation

closes #948

src/bigint.cpp

@@ -86,6 +86,11 @@ static void to_twos_complement(BigInt *dest, const BigInt *op, size_t bit_count)
     size_t digits_to_copy = bit_count / 64;
     size_t leftover_bits = bit_count % 64;
     dest->digit_count = digits_to_copy + ((leftover_bits == 0) ? 0 : 1);
+    if (dest->digit_count == 1 && leftover_bits == 0) {
+        dest->data.digit = op_digits[0];
+        if (dest->data.digit == 0) dest->digit_count = 0;
+        return;
+    }
     dest->data.digits = allocate_nonzero<uint64_t>(dest->digit_count);
     for (size_t i = 0; i < digits_to_copy; i += 1) {
         uint64_t digit = (i < op->digit_count) ? op_digits[i] : 0;

test/cases/eval.zig

@@ -513,3 +513,19 @@ test "array concat of slices gives slice" {
         assert(std.mem.eql(u8, c, "aoeuasdf"));
     }
 }
+
+test "comptime shlWithOverflow" {
+    const ct_shifted: u64 = comptime amt: {
+        var amt = u64(0);
+        _ = @shlWithOverflow(u64, ~u64(0), 16, &amt);
+        break :amt amt;
+    };
+
+    const rt_shifted: u64 = amt: {
+        var amt = u64(0);
+        _ = @shlWithOverflow(u64, ~u64(0), 16, &amt);
+        break :amt amt;
+    };
+
+    assert(ct_shifted == rt_shifted);
+}